Wear part

ABSTRACT

A refurbished knife holder for wood chippers and a method of repairing wear parts are disclosed. The knife holder includes a machined recess and an insert bonded to the holder at the recess and machined to define the surfaces of the holder. The method includes the steps of preparing the wear part and the insert for bonding and bonding the insert to the wear part by an induction heating operation at approximately 1200° F. A silver alloy filler material having approximately 50% silver and a flow point of approximately 1180° F is employed.

BACKGROUND OF THE INVENTION

This invention relates to wear parts and a method for the repair of wearparts and more particularly to the repair of the wear parts of woodchippers.

Conventional wood chippers for reducing slabs, logs or even entire treesto wood chips generally employ a rotatably mounted cutting disc. Thecutting disc is usually formed from a mild steel and may beapproximately five inches thick. A plurality of cutting blades or knivesare mounted adjacent apertures on the cutting disc to be rotatedtherewith. The rotating disc is mounted on a base structure andtypically includes a wear plate mounted to the face of the disc. Thecutting knives which may be from 14 to 28 inches in length are securedto the aperture sides of the disc by a knife holder, counter knife andknife clamp, all of which are bolted directly or indirectly to therotating disc. A housing surrounds the disc and includes an openingthrough which the wood to be chipped may be passed. The opening iscovered by a spout which terminates in a feed plate adjacent an anvil.During rotation of the disc, the cutting knives are passed adjacent theface of the anvil and at such an angle so as to draw in the raw materialto produce wood chips which are expelled through an exhaust chuteusually at the periphery of the housing. The wood chips are generallyemployed in the paper mill industry.

The anvil, the feed plate, the wear plate mounted on the rotating disc,the knife clamp, and the knife holder are all subjected to wear due tothe extreme frictional and impact forces incurred during operation.Since close tolerances are involved with respect to the clearancebetween the stationary and the rotating parts and due to the relativelyhigh rpm at which the massive steel disc rotates, these wear parts mustbe replaced quite frequently to maintain efficient operation of thechipper.

Due to the relatively high cost of each of these individual wear parts,attempts have been made to repair the worn parts for reuse. Since thesewear parts are subject to extreme stresses, are machined for closetolerances with respect to the individual parts to achieve the properclearances between the rotating and nonrotating parts in order to obtainsatisfactory chipping, and the fact that the rotating disc must be keptin balance, prior attempts to repair them have been entirelyunsatisfactory.

For example, it has been proposed to manufacture the wear parts with areplaceable mechanically attached wear plate using bolts or otherfasteners. Due to the shock loads encountered by the wear parts duringuse, the bolts generally fail or loosen. This is an extremely dangeroussituation since the metal parts may be flung from the cutter or chipperat a high rate of speed resulting in possible injury to operatingpersonnel and destruction of the chipper. Further, the use of mechanicalmembers to attach inserts in the repair of the wear elements hasresulted in an imbalancing of the rotating disc, thereby subjecting allparts of the chipper to vibration and subsequent cyclic fatigue.

It has also been proposed to resurface the worn areas by applying afiller material directly to the metal parts as with a metal arc weldingprocess. The weldment was then ground to size. The generally hightemperatures involved in an arc welding process have resulted in warpageand twisting of the wear parts. As a result, the attempted repair istotally unsatisfactory in that the necessary tolerances with respect tothe wear parts and the necessary clearances in the chipper have not beenobtainable.

Finally, attempts have been made to machine a recess in the wear part atthe wear area and fill the recess with a mosaic of tungsten carbideinserts. The inserts are brazed at a temperature of 1600° F to the wearparts. This has been an unsatisfactory method of repair with respect tothe wear parts of the wood chipper, since the differences in the thermalexpansion of the tungsten carbide insert and the base metal of the wearpart have resulted in warpage and twisting of the part upon the attemptto repair it. In both of the latter efforts, the wood chippers eitherexploded or would not operate properly.

It is, therefore, readily apparent that all prior attempts to repair orresurface the wear parts found in wood chippers have been totallyunsuccessful due to the exacting manufacturing tolerances involved, theclose clearances between the rotating and nonrotating parts, thenecessity of maintaining the cutting disc in balance and the fact thatall prior methods attempted have resulted in twisting or warpage of thewear parts.

SUMMARY OF THE INVENTION

In accordance with the present invention, improved refurbished wearparts of wood chippers are provided. Essentially, the wear part is firstmachined to remove material from the worn area and form a recess. Aninsert is formed from a material having a coefficient of expansionsubstantially equal to that of the wear part. The insert is then cut andground to fit the formed recess. The surfaces of the insert and themachined area are then prepared for brazing with a silver solder orequivalent filler alloy metal. The insert is then positioned within therecess and the wear part is heated to a temperature of approximately1200° F to braze the insert in the recess. The repaired part is thencooled and the insert and the interface edges machined so that the partassumes its original tolerances.

The resultant repaired wear part is readily usable at a substantiallylower cost than would be encountered if the part were replaced with anew one. There is thus provided a repaired wear part of a chipper havingwearability at least equal to an original wear part, a method forbonding inserts of a hard surfaced material having a coefficient ofexpansion substantially the same as the wear part to the wear part; anda method by which wear parts of wood chippers may be repairedaccurately, easily and with none of the problems heretofore experiencedwith the prior methods.

The features, advantages and objects of the present invention can bestbe understood by reference to the following description thereof togetherwith the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevation of a wood chipper of the type including wearparts which form the subject of the present invention;

FIG. 2 is a fragmentary enlarged cross section taken generally alongsection line II--II of FIG. 1;

FIG. 3 is an enlarged cross sectional view of a knife holder of thepresent invention and repaired in accordance with the method of thepresent invention; and

FIG. 4 is a flow diagram in block form showing the process employed tomanufacture the wear part shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical wood chipper employing wear parts repairable in accordancewith the method of the subject invention is shown in FIG. 1 andgenerally designated 10. As shown, the wood chipper 10 includes a base12, a protective housing 14 including an inlet chute 16 and an exhaustchute 18. A steel chipper disc 32 (FIG. 2) is secured to a shaft 20. Theshaft 20 is rotatably supported within a pair of spaced journal bearings22. The wood to be chipped enters the machine through the inlet 16 andas best seen in FIG. 2, abuts a vertical anvil 30 supported by an anvilclamp 24 and keeper plate 25. A feed plate 28 extends from the inlet 16and abuts the vertical anvil 30. The rotating steel disc 32 has adisc-shaped wear plate 34 bolted to one face thereof. Disc 32 and plate34 each include apertures 31 and 33, respectively, having configuratededge surfaces and tapered outwardly from the front to the back of thechipper to permit wood chips to pass through the chipper disc and plate.

An arrangement including a knife holder 36, a counter knife 38 and aknife clamp 40 is secured by suitable bolts to the disc 32 adjacentapertures 31 and 33 and serves to retain the knives 26 in position.

In operation, the disc 32 of the wood chipper illustrated in FIGS. 1 and2 is rotated at approximately 850 revolutions per minute by a suitableengine (not shown) coupled to drive shaft 22 to which disc 32 issecured. As the wood is fed into chute 16 and abuts anvil 30, the knivesslice chips from the end of the wood which are expelled throughapertures 31 and 33 and finally from the chipper through chute 18 forcollection. During operation, due to the severe pounding and forcesincluding frictional and impact, the areas immediately adjacent theknives and which come into contact with the wood suffer rapid wear.These wear areas are identified in FIG. 2 by darkened areas. As shown,the feed plate 28, the vertical anvil 30, the spout 16 and the knifeclamp 40 are all subject to wear adjacent the entrance point to thecutter. The wear plate 34, the counter knife 38, and the knife holder 36are all subject to wear adjacent the entrance to the chip slot oraperture 31 formed in the disc 32.

The angle which the knives 26 assume relative to the anvil 30 and thefeed spout 16 is critical for proper operation of the machine. Further,the clearance between the tip of the knife and the face of anvil 30 isalso critical. The knives employed in this type of wood chipper aregenerally fourteen to twenty-eight inches in length depending on theoverall size of the chipper. As the knife holder 36 wears adjacent theinlet to the chip slots (31, 33), the tip portion which abuts thecounter knife 38 has a tendency to peel backwards into the chip slot. Asa result, dirt, sand and wood chips are able to enter the area betweenthe face of the knife holder 36 and the counter knife as well as otherareas between the knife clamp 40 and the knife 26. As the wood chipsbuild up in these areas, the knife may be forced outwardly in thedirection of the anvil 30 ultimately resulting in the knife striking theanvil causing serious damage to the wood chipper. As a result, aninspection of the wear parts of the wood chipper must be made prior toeach operation of the device. If the parts have worn past acceptablelimits, they must be replaced. Replacement with original equipment partsis very costly since the knife holder alone may cost in excess ofseveral hundred dollars. Due to the fact that heretofore no acceptablemethod was available to repair these parts, they were generallydiscarded.

The description of the present invention will be primarily directed to arepaired knife holder 36 (FIG. 3), although the invention is applicableto the remaining wear parts in the wood chipper as well as other metalparts which are subject to wear. As shown in FIG. 3, the knife holder 36includes a vertical surface 44 which abuts the disc 32, an outer surface46, a notched counter knife support surface 48 and knife supportsurfaces 49 and 50. The outer surface 46 is the surface primarilysubject to wear during operation of the wood chipper. The holder alsoincludes a plurality of spaced apertures 47 for receiving bolts 45securing the holder to disc 32 as seen in FIG. 2. The bottom surface 41of the holder abuts against the notched area of disc 32 adjacent chipslot 33 as seen in FIG. 2.

The repaired knife holder 36 includes a machined recess 52 within whichis disposed an insert 54. The insert may be formed from the samematerial as that employed in forming the knife holder 36. Typically, amild steel is employed such as SAE 1020 or 1040 steel. It is preferred,however, that the insert be formed from a hard surfaced material such ascase hardened steel, having a coefficient of expansion substantiallyequal to that of the disc material. The repaired holder will thenpossess a usable life in excess of the original equipment holder.

As more fully described below, the insert 54 and the knife holder recess52 are prepared so that coalescence between the insert 54 and the knifeholder 36 may be accomplished through induction brazing. A silver alloyfiller metal 56 having a melting point lower than the melting point ofthe base metal employed in the steel insert and the knife holder isinterposed therebetween for bonding the insert within the recess of theholder. The counter knife support surface 48 and the outer surface 46then are machined so that the dimensions of the repaired knife holder 36are the same as that of the original equipment holder.

In the first step of the process in accordance with the presentinvention, the wear part 36 is machined along its outer surface to formthe recess 52. In machining out material from the worn area of theholder 36, the amount of metal which must be removed will vary dependingupon the size of the knife holder and the amount of wear to which it hasbeen subject. Next, the insert, formed from a mild, low carbon or casehardened steel of the same general type as that employed with the disc36, is cut and ground to size. These steps are designated 57 and 58,respectively, in FIG. 4. On cutting the inserts, the thickness generallymay vary from 1/8 of an inch to 1/4 of an inch and the width from 1 inchup to 21/2 inches. The length of the insert will be slightly longer thanthe length of the knife holder 36. The insert must assume a slip fitwith the recess. The next step 60 involves grit blasting the matingsurfaces of the insert 54 and the machined out recess 52. This gritblasting operation preferably employs conventional abrasives such ascommercially available 46/70 WSC silicon carbide applied under 100-140psi. This step provides a larger surface area to effect bonding of theinsert to the knife holder. After the grit blasting, the mating surfacesare cleaned with a solution of trichloroethylene and then dried using astream of pressurized air as indicated at 62. Next, a commerciallyavailable, high temperature black flux which comes in paste form isapplied to the mating surfaces of the insert and recess as indicated atstep 64. The flux is employed to dissolve oxides as well as to clean themetal surfaces.

Next, the silver alloy filler metal or solder 56 is cut to fit the areaof the recess 52. Any readily available commercial silver solder alloyfiller material may be employed. It is preferred, however, that thesolder contain approximately 50% silver and have a melting point ofapproximately 1120° F and a flow point of approximately 1180° F. Thesolder is available as rolled sheet stock and is readily cut to size.The solder cutting step is 66 in FIG. 4.

Next, as indicated at step 68, the solder is laid within the recess 52on top of the black flux. Next, as indicated at 70, the black flux isspread over the top of the solder within the recess. Next, the cut andground insert 54 is positioned within the prepared recess 52, asindicated at step 72. The prepared insert and knife holder are thendisposed within the coil of an induction furnace to supply the necessaryenergy to raise the temperature of the insert, silver alloy filler metaland the knife holder. This induction heating step of the parts, asindicated at 74, employs a low frequency, large coil induction furnace.In one embodiment of the invention, a commercially available 50 kilowattTOCO furnace was successfully employed with 20-24 inch wear partstotally within the induction coil. By controlling the current throughthe induction coil and/or the time within the coil, distortion of theknife holder during the repair process is substantially eliminated. Theknife holder is heated to a temperature of approximately 1200° F toensure flow of the silver alloy filler metal for a period of 1 to 2minutes. The use of an insert formed from a surface hardened steelhaving a coefficient of expansion substantially equal to that of thedisc material eliminates uncorrectable warpage or twisting of the holderduring the heating operation and the surface hardened insert has greaterwear resistance than the holder base metal.

Either during the operation of the induction furnace or immediatelythereafter, as indicated at step 76, the insert should be moved slightlywithin the recess to ensure adequate and complete wetting of theinterface between the insert and the knife holder by the silver alloymetal filler. Next, as indicated at step 78, the repaired knife holderis removed from the induction furnace and cooled. This cooling operationpreferably is accomplished while holding the insert down with slight orfinger pressure.

Once the repaired knife holder is cooled to room temperature, the endsof the insert are cut to length by snagging 80. Next, as indicated at82, the repaired knife holder may be placed within a 200 ton press inthe event that any slight distortion occurred during the brazingoperation. Such an operation is capable of correcting up to 1/64 of aninch of distortion which is minimal as compared to the excessive warpingand twisting which occurs in the prior art processes.

Finally, as indicated at steps 84 and 86, respectively, the surfaces ofthe insert are grit blasted and the outer surface and interface sides ofthe insert and the knife holder are ground at 84. The resulting repairedknife holder has dimensions within the tolerances allotted for a newpart. The overall process permits the repairing of the knife holderwithout unacceptable distortions and/or twisting. It has been found thatthe repaired article in fact has an increased life when compared withthe original part. Substantial savings are encountered for users of woodchippers of the type described since it is no longer necessary for themto discard the worn part and replace it with an expensive new knifeholder.

It can, therefore, be seen that the products of and process of thesubject invention substantially alleviate all of the problems heretoforeexperienced with the repair of wearable items. Since warpage andtwisting are eliminated, it is not necessary to perform extensivemachining upon the part which would generally result in the partbecoming undersized in critical areas so that it cannot be used. It isexpressly intended that the foregoing description is illustrative of thepreferred embodiment only and is not to be considered limiting. The truespirit and scope of the present invention will be determined byreference to the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A method of repairing the wear parts of a wood chipper such as a knife holder, comprising the steps of:removing material from the worn area of the wear part to form a recess; forming an insert from a material having a coefficient of thermal expansion substantially equal to that of the material of the wear part, said insert dimensioned to fit within the recess; fluxing the recess; positioning a silver alloy filler in the recess; positioning the insert within the recess; induction heating the wear part and insert so that bonding occurs by brazing between the wear part and insert, said insert being formed from a surface hardened steel, said fluxing step employing a high temperature, paste form black flux; grit blasting the mating surfaces of said insert and said recess prior to fluxing the recess, said silver alloy filler material containing approximately 50% silver and having a flow point of approximately 1120° F, said induction heating step including heating at a temperature of approximately 1200° F for a period of at least one minute; and moving said insert once said heating step is started to insure wetting of the mating surfaces.
 2. A method as defined by claim 1 further comprising the steps of cleaning the mating surfaces of the insert and recess after the grit blasting step; anddrying the mating surfaces.
 3. A method as defined by claim 2 further comprising the steps of:cooling the insert and wear part after the heating step; and clamping the insert to the wear part with light pressure during said cooling step.
 4. A method as defined by claim 3 including the additional step of machining the surfaces of the insert so that the repaired part assumes its original tolerances.
 5. A method as defined by claim 4 including the additional step of machining the interface between the insert and the wear part at the ends of the wear part. 